The present invention generally relates to communication signal processing, and in particular relates to reducing undesired amplitude modulation in generated signal pulses.
Communication signal transmitters, such as the transmitters in cellular radiotelephones or other type of mobile communication device, generate transmit signal information in accordance with one or more defined communication standards or protocols. For example, the ubiquitous Global System for Mobile communications (GSM) standard defines a Gaussian Minimum Shift Keying (GMSK) transmit signal modulation format. GMSK is a form of non-linear continuous phase MSK modulation wherein a Gaussian filter is used to restrict the signal bandwidth.
Although the modulation format is inherently nonlinear, GMSK signals can be approximated as a superposition of amplitude modulated pulses. Thus, “linearized” GMSK signals may be generated using Quadrature Amplitude Modulation (QAM) or, equivalently, Offset Quadrature Phase Shift Keying (OQPSK), based on an in-phase (I) and quadrature (Q) signal pair. One advantage of generating linearized GMSK signals lies with the resultant efficiencies gained by reusing linear modulation signal processing elements.
For example, the Enhanced Data rates through GSM Evolution (EDGE) standard provides a mechanism for increasing data rates in GSM-based networks via the adoption of an 8PSK modulation format. Thus, a GSM/EDGE terminal is required to generate both GMSK and 8PSK signals depending on its mode of operation. Since 8PSK modulation is inherently linear, such terminals must include linear signal processing elements and, thus, adopting a linearized GMSK signal format allows reuse of at some of these processing elements.
However, the adoption of linearized GMSK signal generation is not without its drawbacks. For example, a certain amount of “ringing” is a common side effect of passing the I/Q components of linearized GMSK through the required pulse-shaping filter, i.e., the pulse mask that limits the spectral content of linearized GMSK pulses. That is, the nominally constant-amplitude portions of the generated linearized GMSK pulses in fact have a certain amount of undesired amplitude modulation that represent pulse shaping filter artifacts. These artifacts may or may not be harmful, depending on the limitations of the particular transmitter architecture, and on their magnitude.
More generally, those skilled in the art will appreciate that transmit signal pulse generation, GMSK or otherwise, often yields imperfectly generated pulses that are subject to overshoot, undershoot, ringing, and other pulse generation artifacts. These artifacts may cause any number of problems with respect to transmit signal generation.